Presenilin 1 (PS1) is the catalytic core of the ?-secretase and is mostly studied for its role in APP processing and A? generation. Mutations in PS1 cause early onset familial Alzheimer's disease (fAD). Furthermore, clinical observations suggest that AD patients, particularly with familial AD, have a higher incidence of epileptic seizures (Palop&Mucke, 2009), suggesting that fAD mutations, A?-related or not, could contribute to hyperactivity. In the search for PS1-modulating proteins we discovered an intriguing link between PS1 and GLT-1, a major glutamate transporter in the CNS. We found that PS1 interacts selectively with the GLT-1 transporter in mouse brain, and the interaction occurs in both neurons and astrocytes (Zoltowska et al., 2018). This raises the exciting possibility that PS1 may play a role in GLT1-mediated glutamate uptake. The impaired glutamate transport/neuronal network hyperactivity and A? accumulation in the brain are some of the earliest events in the AD pathological cascade that occur during the early ?preclinical? stage of AD (Masliah et al, 1996; Quiroz et al., 2010; Jack et al. 2009). We propose a mechanistic study focusing on the role of the newly discovered interaction between PS1 and GLT-1 glutamate transporter. Aim 1 will explore in detail the physiological triggers and dynamic nature of the PS1-GLT1 interaction and determine the exact interaction sites. Aim 2 will decipher the functional outcome(s) of this novel interaction between PS1 and GLT1, and establish how manipulations of PS1 and GLT1 (e.g. expression up/down-regulation, inhibitors and activators, fAD PS1 mutations, PS1 hyper-phosphorylation etc.) affect the other protein's function. Aim 3 will validate the physiological relevance of the PS1-GLT1 interaction in vivo by establishing if it is affected in ageing mouse brain and in the brain of AD patients, and whether these interactions can be manipulated by physiological stimuli (e.g., optogenetics) and pharmacologically (repurposing FDA-approved drugs) in a mouse model of amyloidosis. The overall hypothesis is that faulty PS1-GLT1 interactions are an early trigger of the disease, affecting both A? and glutamate transport. We will use biochemical, FRET-based and super-resolution STORM imaging, as well as functional assays to examine the functional crosstalk between the PS1 and GLT1, and its importance for glutamate homeostasis (PS1 effects on GLT1) and A? pathology (GLT1 effects on PS1). Since impaired PS1-GLT1 interaction could contribute to AD pathogenesis in these A?-dependent and A?- independent manners, targeting the interaction may provide novel therapeutic opportunities with dual beneficial effects.